The present invention is generally related to monitoring systems and methods. More particularly, the present invention pertains to monitoring in corrosive environments.
Metal-containing structures are vulnerable to the attack of corrosion. Defenses against corrosion are many and vary in effectiveness. Ultimately, inspections, some of which can be very costly, are typically called for to monitor the progress of corrosion. These inspections are usually scheduled on an established time basis. For example, with respect to aircraft structure, regularly scheduled calendar inspections may be performed (e.g., daily inspections may be performed through visual checks), inspections may be performed based on operating time, and inspections may be based on use of the aircraft and the conditions to which the aircraft has been subjected (e.g., freshwater or saltwater landings, operation in muddy or swampy terrain, washing or after heavy rains, etc.).
Corrosion typically occurs in regions that are subjected to excess moisture or wetted by other fluids. For example, in the fuselage of an aircraft, these areas may include the fuel shelf areas, wheel well shelves/back walls in various aircraft, doors including cargo access and landing gear doors, floors of cargo bays, etc.
Various existing non-destructive inspection methods are available to detect corrosion. These detection methods include visual tests, tap test, electrical resistance probing, electrochemical analysis, ultrasonic, eddy current, x-ray radiography, and acoustic emission with heat.
Further, various corrosion sensors are available. For example, one such sensor is described in U.S. Pat. No. 5,549,803 to Schoess et al., entitled xe2x80x9cSmart Fastener,xe2x80x9d issued Aug. 27, 1996. However, although various corrosion sensors are available, such corrosion sensors typically detect corrosion only after it is already occurring. As such, because corrosion is already occurring, preventive action is more difficult to implement.
Generally, preventive measures are indicated as being necessary by inspections as described above, e.g., daily inspections, calendar-based inspections, etc. However, such maintenance inspections are costly. Therefore, it is desirable to reduce the number of inspections or provide for more optimized time periods between inspection events. For example, in mild corrosive environments, abbreviated inspections may be carried out every 90 days, with in-depth inspections being carried out every 180 days. In comparison, in more severe corrosive environments, such abbreviated inspections may be carried out every 15 days, with in-depth inspections being carried out every 30 days.
However, it is difficult to judge what environment conditions will be within a certain time period, even within a particular geographical location. As such, for example, scheduling in-depth inspections every 30 days in geographical areas characterized by severe corrosive environments even during time periods when such corrosive environmental conditions are not occurring is inefficient. Therefore, calendar-based inspections even when scheduled based on the generalized corrosive environmental conditions of certain geographical regions is inadequate from a cost efficiency standpoint.
The present invention provides a monitoring apparatus and method to monitor the presence of corrosive agents in a spatial volume, e.g., the space under the cargo bay floor of an aircraft. The monitor generates an exposure index, e.g., an index which is intended to provide a condition-based metric that indicates when a corrosion inspection should be performed. The monitoring apparatus and method can be used for various purposes, including, but not limited to, scheduling of inspections on a condition basis as opposed to calendar-based inspection.
A monitoring method according to the present invention for monitoring an environment in which an object is located includes monitoring one or more environmental factors (e.g., chloride ion concentration, pH, humidity, etc.) associated with corrosion of materials in the environment. An exposure index representative of cumulative exposure of the object to the one or more environmental factors is then determined based on the monitored environmental factors.
In one embodiment of the method, the exposure index is indicated to a user, such as by displaying the exposure index, setting off an alarm or LED indicator, etc. Further, such an exposure index may be continuously updated and/or displayed. Yet further, in other embodiments of the method, data representative of the monitored environmental factors is recorded.
In yet another embodiment of the method, the object may be inspected as a function of the exposure index. Such inspection is representative of a condition-based inspection as opposed to a calendar-based inspection.
In addition, in various embodiments of the method, the one or more environmental factors associated with corrosion of materials includes at least one of chloride ion concentration, pH, temperature, and humidity. Further, a measured free potential of a sample material representative of a material of which the object is formed may be measured. The measured free potential may be used to verify the exposure index.
A monitoring apparatus for monitoring an environment in which an object is located is also described according to the present invention. Such a monitoring apparatus includes one or more sensors. Each sensor is operable to detect the presence of at least one environmental factor associated with corrosion of materials and provide a sensor signal representative of the detected environmental factor. A processing unit is operable to receive the sensor signals generated by the one or more sensors. The processing unit determines an exposure index representative of cumulative exposure of the object to the one or more environmental factors as a function of the received sensor signals.
In one embodiment of the apparatus, an indication device is used to provide a user with an indication of the exposure index, e.g., the index is displayed for a user. Further, the apparatus may include memory to store data representative of at least one of the exposure index and/or data representative of the sensor signals. Preferably, the indication device continuously updates the exposure index.
In other embodiments of the apparatus, the sensors may include at least one of a chloride ion concentration sensor, a pH sensor, a humidity sensor, a clocking device for use in determining time of wetness, a sensor to measure free potential of a sample material (e.g., a material of which the object is formed) positioned in the environment.
Another monitoring method for monitoring an environment in which an object is located is also described. The monitoring method includes monitoring at least chloride ion concentration and pH in the environment in which the object is located. An exposure index representative of cumulative exposure of the object to at least chloride ion concentration and pH is determined.
Preferably, in addition to monitoring chloride ion concentration and pH in the environment, humidity and temperature is monitored and time of wetness in the environment is also determined. Based on such environmental factors, an exposure index is determined.